ABSTRACT ? PROJECT 2 Cancer remains the leading cause of death by disease in US children, despite major advances in the last 20 years. Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, and optimizing treatment has improved cure rates to >80%. Improvement in the treatment of childhood ALL has occurred by more effective use of anti-leukemic agents that have been available for decades, not by the introduction of new anti- leukemic agents (with the exception of tyrosine kinase inhibitors that are effective in a minority of childhood ALL patients). Although additional ?targeted agents? hold promise for improving treatment efficacy and reducing toxicity, there are very few available for ALL, and they are always given with conventional chemotherapy, which will remain the mainstay of treatment for decades to come (1, 2). For adults with ALL, cure rates are much lower, with only 40-50% of adults being cured. The biological and pharmacogenomic differences between childhood and adult ALL are largely uncharacterized, representing a major knowledge gap and barrier to progress. Pharmacogenomics has played an important role in improving the efficacy and reducing the toxicity of ALL chemotherapy in children, and holds great promise to further improve treatment for both children and adults as discoveries are translated via precision medicine to optimize ALL treatment. Resistance of ALL cells to chemotherapy, whether present at diagnosis (de novo resistance) or acquired during treatment (acquired resistance) is the major cause of treatment failure in both children and adults. However, the causes of ALL drug resistance are poorly understood. This Center project is focused on elucidating the genomic determinants of de novo (Aim 1) and acquired (Aim 2) drug resistance in childhood and adult ALL, using genome-wide strategies. Project 2 builds on our decades of experience and expertise in leukemia pharmacogenomics, and synergizes with research in both Projects 1 and 3, while capitalizing on the P50 Cores to comprehensively interrogate genome variation in germline and leukemia cell DNA. In Aims 1 and 2, we will use state-of-the-art technologies to identify DNA sequence, methylation, and structural alterations, and mRNA and miRNA expression in leukemia cells that influence de novo and acquired drug resistance. Aim 3 will interrogate mechanisms by which genome variants influence drug resistance. Our research is enabled by our success in obtaining biological samples from children and adults (i.e., leukemia cells, normal leukocytes, nucleic acids), in assessing treatment response, and in defining leukemia cell sensitivity to chemotherapy at initial diagnosis and at relapse. This project capitalizes on decades of established collaboration amongst pediatric ALL clinical investigators and basic and translational scientists who are leaders in this Center, as well as extensive new collaborations with adult ALL investigators and genomics experts across each of the three Projects and our Cores. This is the most comprehensive pharmacogenomic study of ALL drug resistance undertaken in children and adults, positioning us to make important advances in this often fatal disease.